Common applications within the field of electronic circuits for e.g. use in telecommunication tend to use higher and higher frequencies. Today the frequency range is stretching into the GHz range. At those frequencies the properties of the substrates used for building monolithic integrated circuits become more and more important. Conventionally, semi-insulating substrates based on GaAs have been used for monolithic integrated circuits to be used for processing signals having frequencies within the microwave range, such circuits being called MMIC (=Monolithic Microwave Integrated Circuits) and thus having good signal properties for gigahertz frequencies.
Recently, a similar substrate material called MICROX.TM. has been proposed by the company Westinghouse for silicon based integrated circuits, see the article by M. H. Hanes et al., "MICROX.TM.--An All-Silicon Technology for Monolithic Microwave Integrated Circuits", Electron Device Letters, Vol. 14, No. 5, May 1993, pp. 219-221. Silicon wafers of the type Silicon on Insulator, SOI, are used comprising high resistivity substrates having an initial resistivity value of about 10 kohmcm. However, it is difficult to produce silicon having such a high resistivity because of the very low impurity concentration required. Moreover, the very manufacturing process of the integrated circuits and components thereof at the substrate surface may lower this resistivity. Still the proposed material is not as good a semi-insulating material as those materials which can be obtained using GaAs or even InP as base materials, what negatively influences the high frequency characteristics of circuits built from such substrates.
In the paper by Vu Quoc Ho and Takauo Sugano, "Fabrication of Si MOSFET's Using Neutron-lrradiated Silicon as Semi-Insulating Substrate", IEEE-TED 24 (4), p. 487 (1982), a method is disclosed for obtaining silicon having a very high resistivity by irradiating silicon with neutrons, the produced material having a resistivity that is unstable during processing for producing integrated circuits.
Some semiconductor materials have been found to have semi-insulating properties, where these properties can be explained as derived from precipitates within the materials acting as or creating "buried" barriers having overlapping depletion regions. For semiconductor materials based on GaAs the precipitates were found to be nano-particles of As, see the paper by Warren, A. C.; Woodall, J. M.; Freeouf, J. L.; Grischkowsky, D.; and others, "Arsenic precipitates and the semi-insulating properties of GaAs buffer layers grown by low-temperature molecular beam epitaxy", Applied Physics Letters, 24 Sep. 1990, Vol. 57, No. 13, pp. 1331-1333, and for materials based on InP Cu--In precipitates were found, see the paper by Leon, R.P.; Werner, P.; Eder, C.; Weber, E. R., "Structure and thermal stability of Cu--In precipitates and their role in the semi-insulating behaviour of InP:Cu", Applied Physics Letters, Nov. 23, 1992, Vol. 61, No. 21, pp. 2545-2547.